Extensive reserves of petroleum in the form of so-called "heavy crudes" exist in a number of countries, including Western Canada, Venezuela, Russia, the United States and elsewhere. Many of these reserves are located in relatively inaccessible geographic regions. The United Nations Institute For Training And Research (UNITAR) has defined heavy crudes as those having an API gravity of less than 20, suggesting a high content of polynuclear compounds and a relatively low hydrogen content. The term "heavy crude", whenever used in this specification, means a crude having an API gravity of less than 20. In addition to a high specific gravity, heavy crudes in general have other properties in common, including a high content of metals, nitrogen, sulfur and oxygen, and a high Conradson Carbon Residue (CCR). The heavy crudes generally are not fluid at ambient temperatures and do not meet local specifications for pipelineability. It has been proposed that such crudes resulted from microbial action which consumed alkanes, leaving behind the heavier, more complex structures which are now present.
A typical heavy crude oil is that recovered from the tar sands deposits in the Cold Lake region of Alberta in northwestern Canada. The composition and boiling range properties of a Cold Lake crude (as given by V. N. Venketesan and W. R. Shu, J. Canad. Petr. Tech., p66, July-August 1986) is shown in Table A. A topped Mexican heavy crude is included for comparison. The similarities are evident
TABLE A ______________________________________ Analysis of Maya 650.degree. F. and Cold Lake Oil Cold Lake (Lower Grand Rapids Primary Produc- Maya 650.degree. F..sup.+ tion) ______________________________________ % C 84.0 83.8 H 10.4 10.3 N 0.06 0.44 O 0.97 0.81 S 4.7 4.65 CCR 17.3 12.3 % C.sub.7 -Insoluble 18.5 15.0 Asphaltenes Ni, ppm 78 74 V, ppm 372 175 CCR 17.3 -- Boiling Range 75-400.degree. F. 0.62 75-400.degree. F. 1.3 400-800.degree. F. 21.7 400-650.degree. F. 15.2 800-1050.degree. F. 19.0 650-1000.degree. F. 29.7 1050.degree. F..sup.+ 58.71 1000.degree. F..sup.+ 53.8 ______________________________________
TABLE B ______________________________________ Temperature Viscosity, cs (centistokes) ______________________________________ 2.degree. C. (28.degree. F.) Solid 38.degree. C. (100.degree. F.) 4797 54.degree. C. (130.degree. F.) 1137 100.degree. C. (212.degree. F.) 82 ______________________________________
Because of their poor flow properties, the heavy crudes are difficult to produce without assistance, such as steam injection or fire-flooding. Such adjuvants require thermal energy equivalent to 10 to about 30 wt% of the produced oil.
The heavy crudes play little or no role in present-day petroleum refineries. Two principal reasons for this are that they are not amenable to ordinary pipeline transportation, and that because of the high metals and CCR values, they are not readily converted to gasoline and/or distillate fuels without further processing. The progressive depletion and rising cost of high quality crudes, however, create a need for new technology which would inexpensively convert heavy crudes to pipelineable syncrudes, preferably with concomitant upgrading of quality, i.e. ease of conversion to the gasoline and/or distillate fuels which are in heavy demand. Such technology would augment the supply of available crude, and would make it possible for refiners to blend such syncrude with a more conventional feed for catalytic cracking and hydrocracking. The term "syncrude" as used herein means a synthetic crude oil that is a prepared by thermal and/or chemical conversion of a naturally occurring heavy crude oil or a similar substance.
A number of methods have been proposed for decreasing the viscosity of a heavy crude oil so as to improve its pumpability. These include diluting with a light hydrocarbon stream, transporting by heated pipeline, and using various processing options including visbreaking, coking and deasphalting. With most heavy crudes, conventional visbreaking or conventional deasphalting alone cannot give sufficient viscosity reduction. Attempts to reduce the viscosity to the required level by these routes usually lead to an incompatible two-phase product from visbreaking and to a very low yield of deasphalted syncrude from deasphalting.
Visbreaking and coking as practiced in petroleum refineries are conventional thermal processes. Visbreaking is a mild thermal cracking process in which the higher molecular weight components of the heavy oil are cracked to lighter products. Excessive severity in visbreaking is normally undesirable since secondary condensation reactions occur which form incompatible sludge that separates out of the visbroken effluent on dilution with conventional cutter stock. In coking, which is a severe thermal process, the heavy oil feed is heated to cracking temperatures and converted into solid coke and lower molecular weight products which are removed as a vapor. Various coking processes are known, including delayed coking, fluid coking and contact coking and of these, delayed coking is the most common.
Deasphalting by conventional solvent extraction lacks selectivity for many contaminants, resulting in a requirement for a high solvent to oil ratio. The capital cost and energy consumption to handle the solvent in each step of the process are high. The poor selectivity of solvent extraction leads to the production of a large volume of low value product (asphalt).
Mild thermal processing, such as visbreaking, requires relatively little equipment and relatively little in the way of process heat. Visbreaking reduces the viscosity of the product, but not the contaminant content, so the "upgrading" is limited to viscosity reduction. And, furthermore, viscosity reduction is usually not sufficient to achieve pipeline specifications if visbreaking severity is limited to the onset of instability. Visbreaking combined with deasphalting could upgrade the syncrude product but the process then acquires the disadvantages noted above for solvent extraction. More severe thermal processing techniques such as delayed coking are less advantageous. The capital and operating costs of delayed coking are high and the value of the coke product may be less than 1/10 that of the feed (for shot coke) or slightly more (for sponge coke) from low quality residua.
It is an object of this invention to prepare a pipelineable syncrude from a heavy crude oil by severe thermal processing with substantially no production of bulk solid coke. It is a further object of this invention to provide a simple process for preparing a pipelineable syncrude from a heavy crude oil, said process being readily adapted to on-site use and integratable with oil production in a heavy crude oil field. These and further objects will become evident on reading this entire specification including the appended claims.